General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/common/cmd_ambapp.c b/common/cmd_ambapp.c
index 06531f1..bb20ab5 100644
--- a/common/cmd_ambapp.c
+++ b/common/cmd_ambapp.c
@@ -273,6 +273,7 @@
 }
 
 U_BOOT_CMD(ambapp, 1, 1, do_ambapp_print,
-	   "list AMBA Plug&Play information",
-	   "ambapp\n"
-	   "    - lists AMBA (AHB & APB) Plug&Play devices present on the system\n");
+	"list AMBA Plug&Play information",
+	"ambapp\n"
+	"    - lists AMBA (AHB & APB) Plug&Play devices present on the system"
+);